The present invention relates to an apparatus for automatically adjusting an optics of an apparatus for charged particle beam microscopy.
As for automatic correction of an optics of an apparatus for charged particle beam microscopy, there are three known examples as follows. A known example 1 is an automatic alignment unit for a scanning electron microscope disclosed in JP-A-10-92354. In order to adjust a primary electron beam to pass through the center of an objective lens, images before and after varying an exciting current for the objective lens are acquired respectively. After the acquired images are binarized, the displacement direction and the displacement distance of the image caused by misalignment are calculated. An alignment unit is controlled so that the displacement distance vanishes. A known example 2 is an automatic focus corrector for a scanning electron microscope disclosed in JP-A-7-176285. The focused state of a primary charged particle beam is varied. A high frequency component of an image acquired in each focused state is extracted, and an integral value of one screen of the absolute value thereof is recorded. Such integral values in the respective focused states are compared, and a focused state of the primary charged particle beam having a maximum integral value is judged to be an in-focus state. A known example 3 is an automatic focus corrector for a transmission electron microscope disclosed in JP-A-11-138242. When a specimen is located in an in-focus plane, an image is not displaced before and after the incident angle of a primary electron beam is varied. However, if the specimen is out of the in-focus plane, the image is displaced before and after the incident angle of the primary electron beam is varied. This parallactic displacement is analyzed in a method using phase difference of Fourier transform images, and converted into a focal displacement so as to correct the focus.
Problems in the respective known examples will be summarized as follows.
As for the automatic alignment unit as described in the known example 1, there can be mentioned a restriction on applicable specimens. It is difficult to apply image processing including image binarization to a low-S/N, that is, low-contrast image. Such image processing can operate only in a high-contrast field of view for adjusting an optics. In addition, geometrical analysis is easily affected by image deformation caused by a focal variation, so that satisfactory displacement analysis accuracy, that is, alignment analysis accuracy cannot be obtained.
In addition, the displacement analysis used in the known example 1 has no function to express the reliability of its analytic result numerically. Even if analytic impossibility has been caused by image deterioration, an incorrect analytic result is outputted as it is. For example, if a displacement from an optical axis is too large, a specimen may get out of a field of view due to a focal variation so as to cause impossibility of displacement analysis. In addition, if a focal variation is too large, an image may be indistinct enough to cause impossibility of displacement analysis. In the automatic corrector, there is no guarantee that analysis is always carried out correctly. Therefore, means for evaluating the reliability of the analytic result, and a function to interrupt the correction when the reliability runs short are required.
As for the automatic focus corrector as described in the known example 2, dependency of analytic accuracy on specimens can be mentioned as a problem. If a specimen itself has a sharp structure, there can be seen a clear difference in integral value of high frequency components between an image when the specimen is in focus and an image when the specimen is out of focus. However, if a specimen itself does not have a sharp structure, there is little difference in integral value of high frequency components between an image when the specimen is in focus and an image when the specimen is out of focus.
In addition, a large number of data are required in a so-called asymptotic method in which the conditions of an objective lens maximizing the sharpness of an image are searched while the conditions of the objective lens are varied. Data take-in time is limited by physical factors such as the luminance of a charged particle source, the sensitivity of a detector, and so on. Therefore, if data take-in time per unit data is made too short, the S/N ratio of data deteriorates so that analysis becomes difficult. In addition, if the number of data to be taken in is reduced, analytic accuracy deteriorates.
In addition, in the same manner as in the known example 1, there is no index for evaluating the reliability of an analytic result. Therefore, even if a focus cannot be analyzed because there is no sharp structure in the field of view, an incorrect analytic result is outputted as it is.
In the automatic focus corrector for a transmission electron microscope as described in the known example 3, it is assumed that there is a deflector for varying only the incident angle of a primary electron beam. Only in the case where the deflection fulcrum of the deflector agrees with the in-focus plane of an objective lens, only the angle of the primary electron beam incident to a specimen varies when a deflector control value is varied. In any corrector having no deflector with a deflection fulcrum substantially agreeing with the in-focus plane, focal displacement analysis cannot be carried out.